Hi there
I'm working on a model 250 amplifier that came into our shop for repair.
The customer reported that the unit smoked when it was powered up. So far I
have found the following defective components:
1. Q4, Q5, Q8, Q9, drivers open
2. R35, R25, .51 ohm resistors open
3. Q12, Q13 output transistors shorted
4. R27, 681 ohm base resistor for Q7 open
This seems to be an an inordinate amount of component failures in this unit. I've never seen all the drivers on a unit open. Is this a usual occurance on this model?
Also, are these parts still available or can I substitute?
Some other questions; Can I get away with just replacing the two shorted
outputs or should I change them all? Is it critical that these outputs and
the drivers are matched and how tight is the spec on that 681 ohm resistor?
I have checked everything else on the output board and have not found any other problems as of yet however I was wondering if given what I've already mentioned, its felt that I've covered all the bases, or might there be other things I should look at as well?
One other thing: I'm having trying to find replacement outputs and drivers. The outputs are 4 RCA 36892's and the drivers are 2 RCA 38737's and 2 RCA 38736's. All were probably matched I'm assuming.
Anyone know where I might find any of these? Thanks very much for any help and advice.